As presently practiced, silicon ingots are used for silicon wafer fabrication. When used for photovoltaic applications, silicon ingots can be both polycrystalline and monocrystalline. In either case, the ingot needs to be further sawn or shaped into the desired shape after either crystal pulling (monocrystalline) or after solidification from the molten state (polycrystalline) into a block. In the case of monocrystalline pulling, the ingot coming out from the crystal pulling operation is usually cylindrical in shape and has a circular cross section. This cylindrical shape needs to be converted into a near or quasi-square shape by an is operation called “squaring.” During the squaring operation a wire saw cuts ends of the cylindrically shaped ingot in order to end up with a near square ingot. Traditionally this has been done with steel wires together with a slurry that contains an abrasive, such as silicon carbide (SiC).
Similarly for the polycrystalline case, so called silicon “bricks” have to be cut with a wire saw from the block of silicon that results the solidification process. After the ingot shaping operations, the ingot needs to be sliced into wafers. This is also done with a wire saw. As was described previously, conventional wire saws for all these operations comprise steel wires that are used with a slurry that contains the abrasive. The slurry medium in which the abrasive is dissolved is usually ethylene glycol.
However, currently a new type of wire saw is being introduced into the process. The new wire saw comprises a diamond coated steel wire, where the diamond bits are embedded on the steel wire with a nickel coating. For example, a machine that can be adapted to use the new diamond coated steel wire cutting technology is commercially available from the Swiss company MB Wafertec, under the product designations DS 271 and DS 264.
An advantage of the new wire saws (called “diamond wire” saws or DW saws) is that the slurry does not need to contain the abrasive, because the diamond on the wire functions as the abrasive. When using diamond wire saws, there is a need for a new solution that can keep the abraded silicon that is generated during the sawing operation, in solution so that the abraded silicon does not contaminate the photovoltaic substrates or build up on the machinery used with the diamond wire saw. Additionally, the new solution does not need to contain an abrasive and the new solution should be more environmentally friendly with respect to waste treatment and disposal issues than the ethylene glycol solutions used previously.